engleski

Dynamic colour changes of thermochromic leuco dye and liquid crystal based printing inks

Inks that change colour under certain circumstances are becoming increasingly used in applications such as security printing, smart packaging and novelty printing. Thermochromic printing inks change their colour according to a change in temperature. They are characterized by their specific ability to undergo either a reversible or an irreversible colour change. The two types of thermochromic inks are leuco dyes and liquid crystals. Thermochromic leuco dyes usually change from a coloured to a colourless state as the temperature is raised. However, by mixing thermocromic leuco dyes with traditional dyes and pigments an interchange between the two single colours may be achieved. The functional part of thermocromic leuco dyes is in most cases a composite with at least three components: a colour former, a colour developer and a solvent. Its colour changes via two competing reactions, one between the dye and the developer, and the other between the solvent and the developer. The thermochromic effect achieved by liquid crystals is quite different from the effect achieved with leuco dyes. While leuco dyes change from a coloured to a colourless state, liquid crystal inks change colour continuosly throughout the spectrum. This effect is referred to as 'colour-play'. The colour change results from the way light interact with the liquid crystals to produce coloured reflection by interference, and with the variation of the liquid crystal structure due to a temperature change. Our research group is presently working on both types of thermochromic inks. Temperature-dependent optical properties of three commercially available thermochromic screen printing inks were analyzed. In this paper we will give short overview of some thermochromic printing inks with different activation temperature (TA). Research of differences of leuco dye and liquid crystal thermochromic inks will provide knowledge necessary to propose specific guidelines in using thermochromic inks for smart packaging applications.

Thermochromic inks; leuco dye; liquid crystals; colorimetric properties

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